The invention relates to apparatus for assembling a spacer grid of a fuel assembly for a nuclear reactor, and in particular a fuel assembly for a nuclear reactor that is cooled by pressurized water.
The fuel assemblies of water-cooled nuclear reactors and in particular the fuel assemblies of pressurized water-cooled nuclear reactors comprise a skeleton for holding fuel rods in the form of a bundle in which the rods are all parallel to one another.
The skeleton comprises in particular spacer grids that are spaced apart along the length of the fuel assembly, each grid presenting an array of cells through which the fuel rods are inserted. The skeleton of a fuel assembly also comprises guide tubes which are inserted in some of the cells in the spacer grids in the place of fuel rods and which serve to fix the spacer grids in positions that are spaced apart along the length of the guide tubes of the fuel assembly.
Spacer grids are constituted by crossed rectangular metal straps assembled together at right angles along the edges of the cells for receiving fuel rods, which cells are in the shape of rectangular parallelepipeds.
The metal straps of a spacer grid constitutes first and second sets of mutually parallel straps that are assembled together in crossed manner with the straps of the second set being perpendicular to the straps of the first set. The straps are assembled together at each edge by interengaging two straps via slots each extending over half the width of the straps, each of the straps having a set of slots that are spaced apart in pairs by a constant distance in the length direction of the straps.
In this manner, the rectangular cells for receiving the fuel rods (or possibly guide tubes) have four walls each, each wall being constituted by a portion of a strap extending between two successive slots.
Inside each cell for receiving a fuel rod, on two walls of a cell that meet at an angle, there are disposed springs, while on two opposite walls there are disposed rigid bearing dimples so that the rods are held between at least two sets of springs and dimples within each cell. In this manner, the rods are held effectively while still being capable of moving relative to the spacer grids under the effect of thermal expansion inside the nuclear reactor in operation.
The cooling water of the nuclear reactor flows in the longitudinal direction of the fuel rods, which is generally the vertical direction inside a reactor that is in operation.
Within the bundle of rods in the fuel assembly, in order to achieve good distribution and effective heat exchange between the cooling water and the rods, the spacer grids have mixing vanes along the top edges of the rod-receiving cells, which vanes are shaped by being cut out at the same time as the straps are cut out and by being folded towards the insides of the cells.
One of the edges of each strap also has tabs for assembling the spacer grid to guide tubes. The mixing vanes and the tabs are constituted by portions projecting width-wise from the edges of the straps and obtained when the straps are cut out.
The springs and the dimples on the walls of the cells in the grid can be obtained by cutting and embossing the metal of the straps.
In some fuel assemblies, the spring can alternatively be separate fittings applied to the walls of the cells when the spacer grid is assembled.
In general, the straps of spacer grids are made of zirconium alloy. When separate fitted springs are used, the springs can be made of nickel alloy.
The manufacture of spacer grids includes a step of cutting out the straps to a generally rectangular shape from a sheet of zirconium alloy, which cutting-out operation serves to form the mixing vanes and the tabs on the edges of the straps. The straps are also cut at regular intervals to obtain rectilinear assembly slots extending across at least half the width of each strap. Finally, the straps are cut and embossed to make the dimples and possibly also the springs.
A second operation in manufacturing spacer grids consists in assembling the spacer grid by engaging the metal straps of two sets of parallel straps in a disposition where they cross at 90xc2x0. This operation must be performed carefully since it determines whether the proper geometrical characteristics are obtained for the spacer grid. These geometrical conditions themselves have a major influence on the quality of the operation and on the performance of the fuel assembly.
In order to improve the positioning of the straps relative to one another at the time of assembly, it is possible to provide plunged bosses made by cutting and embossing the metal of the straps along the assembly edges of the cells.
Once the spacer grid has been assembled by assembling the straps constituting the inner straps of the grid in a crossed configuration, a framework of outer straps is mounted around the periphery of the grid. Thereafter, the grid is placed in a welding jig and the straps are welded together along the edges of the cells.
A spacer grid can be assembled by assembling together two sets of mutually parallel straps in a manner that is entirely manual, with the straps of a first set of straps being placed in parallel dispositions within a jig and with the straps of the second set of straps being assembled manually one by one by interengaging their slots in the assembly slots of the straps in the first set of straps.
Such a manual operation is lengthy and does not make it possible to guarantee that the spacer grid is of perfect geometrical quality. French patent No. 2 533 352 proposes a method and apparatus for assembling spacer grid straps that enable the straps of the second set of straps to be inserted one by one in the assembly portions of the straps of the first set of straps held in a jig, that operation being performed by using moving equipment for inserting the straps of the second set of straps, one by one, said equipment including means for clamping each strap in a position that facilitates its insertion. The jig is movably mounted on a fixed structure of an assembly apparatus to move in a direction parallel to the longitudinal direction of the straps in the first set of straps that are in position in the jig, and the moving equipment can be moved in a direction that is orthogonal to the travel direction of the jig for the purpose of inserting a strap held by the clamping means.
By moving the jig in the longitudinal direction of the straps in the first set of straps, the assembly portions of the straps in the first set of straps are brought successively into a disposition for engaging a strap of the second set of straps as carried by the moving equipment.
In addition to means for clamping onto a strap in the second set of straps, the moving equipment also comprises a comb for engaging on the straps of the first set of straps to hold them in zones close to the zones where they engage with the strap of the second set of straps while engagement is taking place.
After a strap of the second set of straps has been engaged, it is necessary to retract the means that were clamping onto the strap in the second set of straps that has just been engaged and also to retract the comb that serves to hold the straps of the first set of straps.
Those operations require the strap clamping elements to be put into an open position and the straps of the first set of straps to be moved a little in the longitudinal direction, together with the moving support for the straps of the first set of straps.
The operations of disengaging the comb and the means for clamping onto the straps of the second set of straps are thus relatively lengthy, and at the end of the operation they require the position of the moving support for the straps of the first set of straps to be initialized followed by displacement of the moving support to a new engagement position.
In certain designs of fuel assembly for pressurized water nuclear reactors, the spacer grids have wider springs and longer mixing vanes, and the fixing tabs on the guide tubes are disposed on the edge of each strap opposite from the edge on which the mixing vanes are disposed. Those spacer grids cannot be assembled with the prior art assembly machine because the wider springs of a strap do not leave enough room to pass the mixing vanes of the strap which is being engaged. Because the mixing vanes are longer and because the tabs are disposed on the side of the strap opposite from the side on which the mixing vanes are located, it is necessary for the straps to be moved through a much longer distance for engagement purposes.
To assemble certain spacer grids, there is a manual method like the method described above. The straps of the first set of straps are placed parallel to one another in the grooves of a jig, and the straps of the second set of straps are engaged manually one by one after being fitted with clips for holding down the springs in the thickness of a strap.
Manually assembling the straps is a lengthy operation requiring competent personnel to be employed, and even then there is no guarantee that the geometry of the spacer grid will be perfect.
In addition, the springs are held out of the way in mechanical manner by means of clips, thus exerting stresses on the straps.
The manufacture of straps includes an operation of cutting the straps out from a sheet, which operation serves both to obtain the cutout edges of the straps having the appropriate shapes, and also to obtain internal cutouts in the straps to constitute the slots or notches, the assembly bosses, the dimples, and possibly also the springs. Punching out straps gives rise to stresses and deformation due to portions of a strap being indented. After cutting out, the strap is therefore no longer accurately plane. It is therefore necessary to counter or indeed eliminate this deformation by using appropriate apparatus in order to make it easier, or even possible, to assemble the straps together.
It is also necessary to ensure that the clips that hold the springs down do not add additional deformation while the straps of the second set of straps are being engaged.
No previously known apparatus or method enables the above-recalled drawbacks of prior art assembly apparatuses and methods to be remedied, particularly with certain designs of spacer grid.
The object of the invention is thus to propose apparatus for assembling a spacer grid of a fuel assembly for a nuclear reactor, the grid being constituted by a first set of metal straps of generally rectangular shape that extend parallel to one another, and a second set of metal straps that are rectangular, parallel to one another, and that cross the strap of the first set of straps in planes perpendicular to the straps of the first set, the straps being assembled together by interengagement via slots that are spaced apart at a constant distance in the length direction of the straps, the slots occupying at least half the width of the straps so as to constitute a regular array of rectangular cells, each cell being defined by four walls, each wall being constituted by a portion of a strap that lies between two successive slots, each strap carrying a set of springs projecting from at least one face of the strap over at least a fraction of the portions between the slots of the strap, the assembly apparatus comprising a frame for supporting the straps of the first set of straps so that they are disposed parallel to one another, clamping and engagement means for clamping on each of the straps of the second set of straps in succession and for engaging them with the straps of the first set of straps in position in the support frame, and at least one comb movable between a disengaged position and an engaged position with each of the straps of the first set when in position in the support frame, the comb occupying a position adjacent to a slot for engaging a strap of the second set of straps, said apparatus enabling certain designs of spacer grid to be assembled quickly and automatically while avoiding indentation deformation of the straps during assembly.
For this Purpose:
the support frame is secured to a fixed structure;
the clamping and engagement means and the comb are mounted to move in respective planes perpendicular to the straps of the first set in position in the support frame, on a carriage itself mounted to move in a direction parallel to the longitudinal direction of the straps of the first set on the fixed structure; and
the clamping means comprise two jaws movable in translation in the same direction and in opposite directions under the control of a cam, and the studs for retracting springs of a strap of the second set engaged in the clamping means by pressing them into the thickness of the strap.